The merozoite interacts in a receptor specific manner with the erythrocyte surface and is the stage against which immunity may work to block invasion. Thus, merozoite surface components are of interest for their role in erythrocyte recognition and as antigens for induction of protective immunity. We are now studying one antigen on the merozoite surface that undergoes antigenic variation to understand the molecular basis for this variation. We are studying P. falciparum mutants that use an alternate receptor for invasion of human red cells and continuing the study of the Duffy blood group molecule that is the receptor for P. vivax invasion.